1. Field of the Invention
Stick-form rigid adhesives of the type which are often slidably mounted in a reclosable tube for dispensing and which deposit a tacky film when rubbed onto a receiving surface, have become part of everyday life. They are typically obtained (cf. U.S. Pat. No. 3,576,776) from a water-soluble or water-dispersible adhesive-forming synthetic high polymer, particularly polyvinylpyrrolidone (PVP), dissolved or dispersed in an aqueous-organic liquid phase together with a compatible gel-forming component; the gel-forming component generally includes, in particular, alkali metal or ammonium salts of aliphatic carboxylic acids, especially C.sub.12 -C.sub.22 -acids. On heating the highly tacky, aqueous preparations of the adhesive polymers with small quantities of the gel-forming component to relatively high temperatures, usually above 80.degree. C., the mixture solidifies with cooling into a relatively stiff soap gel in which the comparatively rigid micelle structure of the gel is apparent to a predominant degree. The gelled composition can then be shaped, e.g., formed and handled in stick form, and, if desired, disposed in an applicator, such as a reclosable tube dispenser. When rubbed onto a receiving surface, the micelle structure of the gelled adhesive is destroyed and the solidified composition is converted into an adhesive paste.
2. Discussion of Related Art
Numerous attempts have been made to improve adhesive sticks of this type by modifying the gel-forming component and/or by modifying the solvent-activated adhesive-forming component. For example, U.S. Pat. No. 3,846,363 discloses the use of the reaction product of sorbitol and benzaldehyde as the gel-forming component, while U.S. Pat. No. 4,325,855 describes salts of substituted terephthalic acid amides as gelling agents. According to U.S. Pat. Nos. 3,787,345 and 3,787,346, free long-chain aliphatic acids or esters thereof, rather than alkali metal salts of aliphatic carboxylic acids, are employed as the gel-forming component, and anionic, non-soapy wetting agents are incorporated into the stick to improve rubbing and depositing characteristics. Additionally, U.S. Pat. No. 4,011,311 teaches the use of a reaction product of an aromatic diisocyanate with a mono- and/or dialkanolamine as gel-forming agent.
Despite all these proposals, the oldest form of adhesive sticks, i.e., those based on soap gels such as described in U.S. Pat. 3,576,776, to Muszik, et al., issued Apr. 27, 1971 and incorporated herein by reference, remain by far the most widely used. In a particularly popular embodiment of this technology, a solution of PVP in an aqueous organic solvent mixture is converted into a soft-rubbing adhesive stick by incorporation of alkali soaps of aliphatic carboxylic acids as the gelling agent, followed by gelling of the mixture. These adhesive sticks are, however, susceptible of improvement in several respects. For example, the application of the adhesive to the substrate material from these sticks is often not as easy as is desirable, with the result that rubbing of the adhesive stick onto fragile substrates such as thin paper cannot be readily accomplished. Further, the adhesive layer formed frequently has an inconveniently short open time as compared to similar commercial products, losing tackiness rapidly and hampering careful alignment of the surfaces to be bonded.